PROJECT SUMMARY The Middle East Respiratory Syndrome Coronavirus (MERS-CoV) is a zoonotic virus that can cause fatal disease in patients with underlying comorbidities. Further recognition of this respiratory syndrome and prevention strategies will require a small animal infection model as well as an additional understanding of the virus. This PPG describes a mouse model of MERS-CoV disease. In this model, the viruses causing disease are adapted variants, specialized for mouse lung infection. By contrast, non- adapted MERS-CoVs cause infection in the mouse but do not cause disease. The central hypothesis of this subproject (PPG2) is that mouse-adapted variants can efficiently enter host cells through pathways that are not available to the non-adapted viruses. To address this hypothesis, recombinant MERS-CoVs will be constructed and evaluated to determine whether mouse-adaptive mutations in the cell entry-mediating viral spike proteins correlate with efficient mouse lung infection. Surrogate MERS-CoV pseudo-viruses will be constructed and evaluated to address the focused hypothesis that mouse adapted variants mediate an ?early? plasma-membrane cell entry that is unavailable to non-adapted viruses. The project will dissect mechanisms by which spike proteins mediate early cell entry through plasma membranes versus late cell entry through endosomes. The basis for selection of early versus late cell entry will be determined by identifying host cell factors promoting or restricting either pathway. This project will also identify appropriate antiviral strategies that operate by preventing early and late virus-cell entry. The rationale for all of these aims is that additional understanding of MERS-CoV cell entry pathways will identify correlates of robust infection and disease, and will also provide insights on the best ways to prevent infection and disease with innovative virus entry inhibitors.